This invention is concerned with a signal control system for railroad car brakes in which the following technique is used. The electric brake power control signal is converted into a first air pressure. At the same time, the effective electric brake equivalent signal, which corresponds to the actual electric brake power, is converted into a second air pressure. The air brake is controlled according to the difference between the first air pressure and the second air pressure, which is indicative of the insufficiency of the electric brake.
In this brake control technique for railroad cars, the present system features the following: The insufficiency signal is obtained by subtracting the effective electric brake equivalent signal from the brake power control signal. The variation in this insufficiency signal is detected. When it decreases, a correction signal is added to the equivalent signal.
The present invention is suitable for use in a system for controlling the air brake for railroad cars where both air brakes and electric brakes are used. The air brake provides braking power to supplement the electric brake power when sufficient electric brake power is not obtained for a certain brake power control signal. Such a system is known from the official Japanese patent applications Nos. 60-31162 and 60-166671, and is well known.
In the known arrangement shown in FIG. 4, SB1, SB2, and SB3 denote electrical leads which transmit electric brake power control signals to each control device of a railroad car. The signal consists of three-bit digital data.
Compressed air is supplied to the magnetic valves MV1, MV2, and MV3 from the compressed air supply MR and the variable load valve VLV which is connected to the air spring. The pressure is adjusted according to the car load. These magnetic valves are activated or deactivated (opened or closed) according to the aforementioned brake power control signal, supplying or draining compressed air to/from the relay valve RV which will be described later.
At the same time the brake power control signal is converted into an analog signal by the digital analog converter DA. The brake power setting device BS adds a load signal to the above converted signal according to the car load. The resultant signal is transmitted to the electric brake device EB.
This electric brake device EB works following the above brake power control signal, giving electric brake power to the car.
The effective electric brake equivalent signal, which corresponds to the actual electric brake power, is transmitted to the electro-pressure conversion valve EP through the amplifier AM.
The electro-pressure conversion valve works in the following way: When the colenoid 10 is activated, the pull on the supply exhaust valve control rod 11 opens the supply valve 12. The balance position 14 is pushed up due to the increase of air pressure in the output space 13. When the forces on the two sides of the balance piston 14 are equal, the supply exhaust valve control rod 11 and the supply valve 12 will change to the lap state and the pressure in the output space 13 will be maintained. The compressed air in this output space 13 is supplied to or exhausted from the relay valve RV.
In the relay valve RV, the compressed air supplied to the diaphragm spaces C1, C2, and C3 through the magnetic valves MV1, MV2, and MV3 works to push up the supply exhaust valve control rod 21 through corresponding diaphragm pistons S1, S2, and S3. In contrast, the compressed air supplied to the diaphragm space C4 through the electro-pressure conversion valve EP works in the opposite direction.
In short, the brake power control signal is converted into the first air pressure by the diaphragm spaces C1, C2, and C3, the diaphragm pistons S1, S2, and S3 in the relay valve RV, and the magnetic valves MV1, MV2, and MV3. The output pressure of the electro-pneumatic conversion valve EP is the second air pressure which is converted from the equivalent signal. This second air pressure is subtracted from the first air pressure in the relay valve RV.
When the electric brake power is not sufficient for the given brake power control signal, the supply-exhaust valve 21 opens the supply valve 22 in the relay valve RV. Air pressure in the output space 23 increases. This pressure pushes down the balancing piston 24 until the forces on both sides of the balancing piston 24 are equal. In this condition, the supply/exhaust valve control rod 21 and the supply valve 22 are in lap state and the pressure in the output space 23 will be maintained. This output pressure is applied to the brake cylinder BC which is part of the air brake device.
In summary, the subtraction of the second air pressure from the first air pressure is achieved in the relay valve RV. The air brake device is controlled following the result of the subtraction.
The following problems are pointed out for the conventional air brake control system described above.
The first problem arises when the electric brake device EB fails to respond to the given brake power control signal, giving the equivalent signal of zero. In other words, when electric braking power is absent and the air brake power is supplying all the brake poweor to the car, the pressure in the brake cylinder P3 is controlled with the electric valves MV1, MV2, MV3, and the relay valve RV. In this case, as shown in FIG. 5, the brake cylinder pressure P3 becomes higher for a decreasing brake power control signal than for an increasing signal due to the hysteresis which is caused by the friction in the relay valve RV and other effects. As a result, the total brake power is different from the case where the electric brake device EB works with full efficiency. This means that one cannot get consistent brake power for the same brake power control signals. Not only does the consistency in the stopping distance of the train suffer, but the ride will become uncomfortable for the passengers because of the uneven cooperation of the electric brakes with the air brakes. Secondly, when the air brake supplements the electric brake for the insufficient power for a brake power control signal, a similar problem also occurs. When the total brake load is shared by the electric brake power and the air brake power, the electro-pressure conversion valve EP is also activated by the equivalent signal which corresponds to the actual electric brake power. Hysteresis in the output pressure P2 also occurs in the electro-pressure conversion valve EP due to friction and other effects. This hysteresis is added to the hysteresis in the relay valve RV. As shown in FIG. 6, the hysteresis of the brake cylinder pressure P3 is even larger in this case.